Silver halide photothermographic imaging materials, often referred to as `dry silver` compositions because no liquid development is necessary to produce the final image, have been known in the art for many years. These imaging materials basically comprise a light insensitive, reducible silver source, a light sensitive material which generates silver when irradiated, and a reducing agent for the silver source. The light sensitive material is generally photographic silver halide which must be in catalytic proximity to the light insensitive silver source. Catalytic proximity is an intimate physical association of these two materials so that when silver specks or nuclei are generated by the irradiation or light exposure of the photographic silver halide, those nuclei are able to catalyze the reduction of the silver source by the reducing agent. It has been long understood that silver is a catalyst for the reduction of silver ions and the silver-generating light sensitive silver halide catalyst progenitor may be placed into catalytic proximity with the silver source in a number of different fashions, such as partial metathesis of the silver source with a halogen-containing source (e.g., U.S. Pat. No. 3,457,075), coprecipitation of the silver halide and silver source material (e.g., U.S. Pat. No. 3,839,049), and any other method which intimately associates the silver halide and the silver source.
The silver source used in this area of technology is a material which contains silver ions. The earliest and still preferred source comprises silver salts of long chain carboxylic acids, usually of from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of like molecular weight have been primarily used. Salts of other organic acids or other organic materials such as silver imidazolates have been proposed, and British Pat. No. 1,110,046 discloses the use of complexes of inorganic or organic silver salts as image source materials.
In both photographic and photothermographic emulsions, exposure of the silver halide to light produces small clusters of silver atoms. The imagewise distribution of these clusters is known in the art as the latent image. This latent image generally is not visible by ordinary means and the light sensitive article must be further processed in order to produce a visual image. The visual image is produced by the catalytic reduction of silver which is in catalytic proximity to the specks of the latent image.
As with conventional photographic silver halide, photothermographic emulsions are naturally sensitive only to the blue, violet and ultraviolet portions of the electromagnetic spectrum. The natural sensitivity is also relatively weak at those wavelengths. Dyes which have been used to spectrally sensitize photographic emulsions have been used with reasonable success to spectrally sensitize photothermographic emulsions. This is accomplished by adding the dyes to the emulsion before, during, or after formation or addition of the silver halide component.
The dyes used for spectral sensitization of photographic silver halide emulsions have found only moderate utility in photothermographic emulsions, particularly those used to sensitize in the red. This reduced utility is not with respect to potential sensitizing efficiency, but rather is with respect to the critical effects of concentration variations of the dyes. What would ordinarily be considered as insignificant variations in dye concentrations, .+-.15% from optimum concentrations, can have dramatic and adverse effects on the sensitometry of the photothermogrpahic emulsion. Minor variations in concentrations which can result from insufficient mixing, variations in supply rates, evaporation and other variables can cause fog, thermal instability or shelf life instability.
It would be desirable to find sensitizing dyes, particularly for the red portion of the electromagnetic spectrum, which would not be so concentration sensitive and would allow more manufacturing latitude.